New approaches are required to meet the constantly increasing demand for high data rates in cellular networks. One challenge for network operators is determining how best to evolve their existing cellular networks to meet the requirement for higher data rates. A number of approaches are possible, including: (a) increasing the density of existing macro base stations; (b) increasing the cooperation between macro base stations; or (c) deploying smaller base stations in areas where high data rates are needed within the macro base station grid.
The last option, the use of smaller base stations among a grid of larger, macro base stations is referred to in the related literature as a “Heterogeneous Network” or “Heterogeneous Deployment.” The “layer” of smaller base stations is termed a “micro” or “pico” layer.
Building a denser macro base station grid while simultaneously enhancing the cooperation between macro base stations can be used to meet the requirement for higher data rates. However, such an approach is not necessarily an attractive option, because of the costs and delays associated with the installation of macro base stations. Such costs are especially significant in urban areas. Further, dense deployments of macro base stations leads to significantly higher amounts of signaling because of frequent handovers for wireless communication devices moving at high speeds through the denser macro layer grid.
In contrast, a macro layer grid of “normal” density can serve those devices, equivalently referred to as “users,” moving at high speed, as well as provide wider areas of service where the demand for high data rates is less, while in the same heterogeneous network, a micro layer grid of smaller base stations can be deployed in service areas having a higher density of users requiring high data rates. The service areas of these smaller base stations may be regarded as “hotspots” within the larger macro service areas. The deployment of small base stations within an already existing macro layer grid is an appealing option at least in some circumstances, because smaller base stations generally are more cost-efficient than macro base stations, and their deployment times are generally shorter as well.
FIG. 1 shows the basic principle of heterogeneous deployments. In particular, FIG. 1 depicts a network 10 on the left side of the arrow, having a number of macro base stations 12, each defining a cell 14, which may be understood as a geographic area of service for a given set of wireless resources—e.g., the intersection or combination of a particular carrier frequency and a particular geographic region. The same network 10 appears on the right side of the arrow, except that the network 10 is now enhanced with a micro layer grid of smaller base stations 16. Each smaller base station 16 provides service in a corresponding micro or pico cell 18. These micro cells 18 appear as hotspots within the larger macro cells 14, and may be understood as sub-regions within the network 10 that are tailored for higher density usage and/or for higher data rate services.
One feature that is associated with heterogeneous network deployments is “muting,” or Time Division Mode (TDM) for transmissions from certain network nodes. Such nodes include the macro base stations 12 and/or the micro or pico base stations 16. For example, in heterogeneous network systems featuring “Open Access” (OA) pico base stations, the macro layer mutes its transmissions during certain Transmission Time Intervals (TTIs), so as not to interfere with users connected to the pico base stations. Such muting of macro base stations is necessary, to avoid creating high interference to users in the extended range of pico cells in cases where the pico cells extend their coverage by using specific cell selection offsets. (Here, it will be understood that the “macro layer” refers to the macro base stations 12 and their operations, while the “micro layer” or “pico layer” refers to the micro or pico base stations 16 and their operations.)
One or more of the pico base stations 16 may be “Home eNodeBs” or other type of low-power access point that often is assigned to and used by a particular subscriber or group of subscribers within a particular premise, such as a home or business. Home eNodeBs, also referred to as HeNBs, use Closed Subscriber Groups (CSGs) to restrict access to certain user equipment (UEs) or other wireless communication devices, so that only authorized devices can gain network access through a given HeNB. These CSG-based HeNBs mute their transmissions in certain TTIs, to avoid causing downlink (DL) interference to nearby users that are connected to a macro base station 12 rather than to the proximate HeNB. In an example case, imagine a UE that is nearby a given HeNB but not part of the CSG defined for that HeNB. In such a case, the UE is within signal range of the HeNB but is not permitted to use it, and therefore must connect to the network 10 through the macro base station 12 that also provides overlapping service coverage in that area.
The TD mode of transmission is also used for inbound relaying for relay nodes (RNs) and relay-supported users. In such operation, the relay is not muting its transmission to users, but the relay at such time instants listens for transmissions from its supporting macro base station 12. In Long Term Evolution (LTE) based networks, the supporting macro base station 12 is an eNodeB or eNB, and is commonly referred to as a “donor” eNB (DeNB). Although this TD mode of transmission of neighboring/overlapping cells is currently specified only for heterogeneous network deployments, in the future the TD mode of transmission may be applied for homogeneous networks as well.
HeNBs and other types of pico base stations 16 are often placed in areas of poor macro cell coverage, such as at the borders between macro cells 14. Consequently, the use of TD mode transmissions by pico base stations 16 generates highly varying and “bursty” other-cell interference levels within the signal ranges of the pico base stations 16.
Typically, CSG Home eNBs, OA pico eNBs and relay nodes are placed at locations of poor macro layer coverage, close to macro cell borders for example, and TD mode of transmission therefore results in nearby UEs experiencing other-cell interference that is highly varying and bursty in nature. Further, TD mode transmissions from macro base stations 12 cause similarly, bursty and highly varying levels of other-cell interference for UEs that are within range of such TD mode transmissions but are served by other macro base stations 12.
One approach to alleviating the effects of interference variance focuses on a relay node using its DeNB to notify neighbor cells with respect to the transmission patterns used by the relay node. The notification allows a neighbor cell, for example, to avoid transmitting or otherwise allocating air interface resources to its cell-edge users at times when the relay nodes in the other cell are transmitting or receiving. This approach may be regarded as a type of collision avoidance, and there are several key assumptions underlying this type of collision-avoidance operation: (a) there is a means of communication between neighboring macro base stations 12, e.g., the “X2” interface that interlinks eNBs in an LTE-based wireless communication network; and (b) not all cell edge users are affected by relay-node or other low-power-node (LPN) transmissions.
The assumption of inter-base-station communications generally holds true in LTE networks, for example, at least if the involved eNBs belong to the same network operator. However, eNBs belonging to different network operators generally are not connected. Moreover, other types of network transceivers, such as HeNBs, may employ muting schemes, without necessarily notifying the closest macro eNBs. Hence, even if X2 or other inter-node interfaces are available (such as the “S1” interface between an eNB and a supporting core network entity), it is not always given that neighbor eNBs are notified of the transmission patterns of LPNs that may affect their transmissions or those of their users. Problematically, simulation results have shown that a major source of other cell-interference in conventional heterogeneous networks (without any modifications in cell selection) are macro base stations 12 and pico base stations 16 (or other LPNs), at least for users operating close to such nodes.